1. Field of the Invention
The present invention relates to a food container made of a polyolefin resin composition having a low level of offensive odors and discoloration.
2. Description of Related Art
It is a common practice to incorporate a polyolefin with an inorganic filler for the improvement of its stiffness, impact resistance, heat resistance, etc., and some of the filled polyolefins find use as food containers. However, the filled polyolefin poses problems when used for food packaging because of its offensive odors, undesirable color, and decreased heat resistance. The offensive odors are noticeable especially when a full food container is sterilized at a high temperature or heated for cooking in a microwave oven. For this reason, there have been proposed several methods for eliminating offensive odors from food containers of polyolefin resin composition. For example, Japanese Patent Laid-open No. 179943/1988 discloses a food container made of a resin composition composed of (1) 100 parts by weight of a composition composed of 90-20 wt % of polyolefin resin and 10-80 wt % of talc and/or calcium carbonate and (2) 0.05-5 parts by weight of an antistatic agent (such as a polyhydric alcohol ester of fatty acid) and/or a slip agent (such as an unsaturated fatty acid amide). The food container, however, is still unsuccessful in the complete elimination of offensive odors and discoloration.
In order to overcome the above-mentioned disadvantages involved in the prior art technology, the present inventors carried out a series of researches which led to a finding that a food container having a low level of offensive odors and discoloration can be obtained from a polyolefin resin composition incorporated with an inorganic filler and a specific phenolic antioxidant and a slip agent and/or antistatic agent.
The investigation by the present inventors revealed that a food container made of a polyolefin composition incorporated with an inorganic filler and an antioxidant gives off offensive odors and becomes discolored when the composition undergoes high-temperature mixing or thermoforming or when the food container is heated. The offensive odors and discoloration result from the reaction of the inorganic filler with the polyolefin and/or antioxidant, which deteriorates the polyolefin and/or decomposes the antioxidant.
As the result of the investigation, it was found that it is possible to eliminate the offensive odors and discoloration almost completely form the food container, if the polyolefin composition is incorporated with (1) a specific high-performance phenolic antioxidant which prevents the deterioration of polyolefins and hardly reacts with inorganic fillers, or gives off no offensive odors even when decomposed, and (2) a specific compound which suppresses the reaction of the phenolic antioxidant with the inorganic filler. This finding is the basis on which the present invention was completed.